Hi, Adan,
So were you guys really using (100%) propane stoves? The ones with the big green cylinders that have 465g worth of propane? Or were you using backpacking type canister stoves (Pocket Rocket, Jetboil, etc). I think you probably were using backpacking type unless you were car camping.
15F to 20F is about the bottom of the temperature range where typical upright backpacking type canister stoves just don't work well anymore. I've got
an article on Seattle Backpackers Magazine about how to (safely!) extend their range a bit that you might want to check out.
You are cutting it a little bit close if you go out into weather that's much colder than maybe 10F. Personally, I'd want to switch to
a remote canister stove, particularly for a group. Cooking for a group on a Trangia (or any alcohol stove) in cold weather is going to be slooooow. Remote canister stoves aren't necessarily cheap, although you might be able to pick one up at a used gear sale. You could also use a white gasoline stove (like a Whisperlite), but those are a little bit heavier. You also have to learn how to prime a stove if you use white gas, but that's no big deal after a few tries. White gas stoves are a lot easier to find on eBay and such (I have a couple of extras if you're interested).
As for the adapters, you can contact
Bluewater Stove Restoration and see if they have any available. Tell 'em that "Hikin' Jim" sent you. He makes them by hand; availability is limited. I'm not sure how busy he is right now, but now is a good time to hit him up. In the summer he's out running rivers.
HJ